1. Field of the Invention
The present invention relates to a cylinder head cooling passage structure for cooling a cylinder head of an Overhead Cam (hereinafter referred to as xe2x80x9cOHCxe2x80x9d) type engine with a coolant.
2. Description of the Related Art
In general, a cooling passage structure of a water-cooled engine is as follows. Water passages are formed in a cylinder and a cylinder head such that the water passages communicate with each other. Cooling water is fed from the water passage of the cylinder to the water passage of the cylinder head.
FIG. 4 is a transverse sectional view showing an OHC V-type engine disclosed in Japanese Laid-Open Patent Publication No. Hei. 10-47057. A cooling passage structure of the OHC V-type engine includes a water passage 150 formed in a cylinder and a water passage 110 formed in a cylinder head. Cooling water introduced from the water passage 150 of the cylinder to the water passage 110 of the cylinder head passes through a water gallery 113 and is discharged to outside of the cylinder head. A cooling water pipe 100P is connected to a cooling water discharge opening 112 communicating with the water gallery 113 of the cylinder head. The cooling water is introduced into the cooling water pipe 100P and returned to a radiator. The cylinder head is provided with an intake valve and an exhaust valve. An intake port 121 and an exhaust port 131, respectively associated with the intake valve and the exhaust valve, are formed in the cylinder head. The water gallery 113 is situated below the intake port 121.
The water gallery is a space into which the cooling water which has finished heat exchange with the cylinder and the cylinder head is finally gathered. Through the water gallery, the cooling water is discharged to outside of the cylinder head. Accordingly, it is required that the water passage of the cylinder head be designed to allow the cooling water to be gathered into the water gallery as smoothly as possible.
The cooling water or coolant, which has finished heat exchange with the cylinder and the cylinder head, has a low density and tends to move upwardly as the temperature increases. Bubbles can become mixed in the cooling water or coolant. It is therefore desirable to place the water gallery as high as possible in the water passage.
In a desirable cooling passage structure for performing well-balanced cooling of the cylinder head, the cooling water which has cooled high temperature portions such as portions around an ignition plug, an exhaust port, and an exhaust valve, without nonuniform flow of the cooling water, is gathered into the water gallery in the vicinity of the intake port where temperature is relatively low and then discharged to outside. By the way, in the conventional V-type engine shown in FIG. 4, the intake port 121 is placed at a position higher than the position of the exhaust port 131, the water gallery 113 is provided at an uppermost portion of the cooling passage structure in the cylinder head, and the cooling water discharge opening 112 and the cooling water pipe 100P are provided upwardly of the water gallery 113. However, in the cylinder head cooling passage structure of FIG. 4, when an attempt is made to provide the water gallery 113 at a higher position in the flow of the cooling water, the water gallery 113 cannot be placed at a sufficiently higher position because of limitation due to placement of the intake port 121. Still, if the water gallery 113 is forcibly provided at the higher position, the cylinder is made larger and it is difficult that the cylinder head is made compact. When an inclination angle of the cylinder is reduced, it is difficult to provide the water gallery 113 at the uppermost portion of the cooling passage.
When the cooling water discharge opening 112 communicating with the water gallery 113 is formed forwardly or rearwardly of the intake port 121 in the direction orthogonal to the cut-away surface as shown in FIG. 4 rather than immediately below the intake port 121, cooling of the cylinder head becomes ill-balanced.
Under the circumstances, an object of the present invention is to provide a cylinder head cooling passage structure of an OHC-type engine which is capable of performing well-balanced cooling of a cylinder head, improving cooling efficiency by providing a water gallery at a high position and making the cylinder head compact.
According to the present invention, there is provided a cylinder head cooling passage structure of an overhead cam type engine comprising: a cylinder head; two intake valves; and an exhaust valve, wherein the two intake valves are provided in the cylinder head such that the intake valves are substantially symmetric with respect to a plane that includes a center axis of a cylinder of the engine and is orthogonal to a crank shaft; the exhaust valve is provided in the cylinder head such that the exhaust valve is situated on an opposite side of the intake valves with respect to a plane that includes the center axis of the cylinder and is in a longitudinal direction of the crank shaft, intake ports associated with the intake valves and an exhaust port associated with the exhaust valve are formed in the cylinder head such that the intake ports extend in a direction substantially orthogonal to the crank shaft and reach one end side of the cylinder head and the exhaust port extends in the direction substantially orthogonal to the crank shaft and reaches the other end side of the cylinder head, a coolant passage is formed in the cylinder head, for passing coolant therethrough, the coolant passage has a coolant inflow opening formed in a bottom face of the cylinder head that is joined to a cylinder block of the engine and a coolant discharge opening formed in an outer face of the cylinder head, a water gallery is formed in the coolant passage such that the water gallery communicates with the coolant discharge opening, the coolant which has finished heat exchange with the cylinder head being gathered into the water gallery, and the water gallery is formed above the intake ports in a direction of the center axis of the cylinder.
According to the cylinder head cooling passage structure described above, the water gallery is formed in the cylinder head such that it is situated at a higher position which is on the intake port""s side. Therefore, the coolant which has an elevated temperature after finishing heat exchange with the cylinder head and has a tendency to go upwardly, is naturally gathered into the water gallery. Likewise, bubbles mixed into the coolant are naturally gathered into the water gallery.
In addition, since it is not necessary to place the intake port at a higher position for the purpose of placing the water gallery at a higher position, the cylinder head can be compactly designed.
Further, since the coolant discharge opening is formed above the intake port, the position of the coolant discharge opening is not restricted by the intake port. As a result, the coolant discharge opening can be provided at a suitable position.
It is preferable that the water gallery is formed such that it spans the two intake ports, and the coolant passage is formed so that the coolant passes through a portion between the two intake ports and outside of the two intake valves and is gathered into the water gallery, because cooling of the vicinity of the intake port can be well-balanced.
The coolant discharge opening may be formed above the water gallery and on a substantially center line of the two intake valves.
Also, it is preferable that the water gallery is formed between the intake port and a side wall of a chamber formed in the cylinder head for accommodating an upper portion of valve stems of the two intake valves, the side wall being closer to the intake port, because the cylinder head can be more compactly designed.
Moreover, it is preferable that the center axis of the cylinder is inclined such that the intake valves are situated at a position higher than the position of the exhaust valve. Thereby, the water gallery is positioned at a higher position, which enables the coolant with elevated temperature or bubbles to be easily gathered into the water gallery.
These and other aspect and advantages of the invention will become apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings.